Two families of anticancer drugs have been shown to be topoisomerase II inhibitors: DNA intercalators, such as amsacrine (m-AMSA), adriamycin (ADR) and ellipticinium (2-Me-9-OH-E+), and demethylepipodophyllotoxins, such as etoposide (VP-16) and teniposide (VM-26). Topoisomerase II (topo II) is a ubiquitous enzyme involved in chromatin structure regulation by controlling both DNA topology (supercoiling, catenation, etc.) and chromatin loop attachment to the nuclear matrix. Topo II is important in DNA replication, transcription and chromatin packaging. Drug inhibition stabilizes topo II-DNA intermediaties, which can be detected as protein-associated DNA strand breaks. We have shown previously that rapidly proliferating and S-phase cells are more drug sensitive than quiescent cells. Now, we have established that newly replicated DNA forms more drug-induced topo II-induced topo II-mediated DNA breaks than bulk DNA and that replicating DNA strands become linked to topo II after they have reached replicon size. We have also investigated the relationship between drug-induced topo II-mediated DNA breaks and cyctotoxicity. A clear correlation is observed within drugs of a single chemical group (acridines, demethylepipodophyllotoxins). Finally, we are trying to identify the biochemical topoisomerase modifications that account for drug resistance in cells resistant to topoisomerase II inhibitors.